1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an apparatus for forming an image by using a plurality of light beams.
2. Related Background Art
Of such apparatus, a color image forming apparatus of a recent electronic photograph type has a plurality of image forming units in order to speed up the image forming operation. Various methods have been proposed for sequentially transferring different color images to a recording medium held on a transport belt.
An apparatus having a plurality of image forming units has the following problems. Because of mechanical precision and the like, a change in motion of a plurality of photosensitive drums and a transport belt, a change in motion relation between a circumferential surface of each photosensitive drum and the transport belt at a transfer position of each image forming unit, and the like occur differently at each color. Therefore, when images of different colors are superposed, color aberration (position misalignment) occurs.
There is an error of an optical distance between a laser scanner and a photosensitive member of each image forming unit. If this error of each image forming unit is different, there is a difference of a main scan magnification of a beam on the photosensitive drum so that the color aberration (position misalignment) occurs. If there is a position displacement of a laser scanner and a photosensitive member of each image forming unit relative to the beam scan direction (hereinafter called a main scan direction) and this position displacement is different at each image forming unit, color aberration (position misalignment) appears on the final images.
In order to reduce the color aberration (position misalignment) to be caused by a difference of the main scan magnification, a method of correcting the main scan magnification has been proposed (e.g., JP-B-06-57040) in which as shown in FIG. 1, a video clock generator for video signals is provided for each color, and the frequency of the video clock for each color is independently changed and adjusted to correct the main scan magnification.
With reference to FIG. 1, a conventional method of changing a video clock frequency of an image forming apparatus will be described.
FIG. 1 is a block diagram showing the structure of a PLL (phase locked loop) for changing the frequency of a video clock signal of a conventional image forming apparatus described in JP-B-06-57040.
In FIG. 1, a crystal oscillator 1007 generates a clock signal 14 at a frequency fin. A 1/M frequency divider 1002 divides the frequency of a clock signal 14 output from the crystal oscillator 1007 by M. A 1/N frequency divider 1006 divides the frequency of a clock signal (video clock) 15 output from a voltage controlled oscillator 1005 by N. A phase comparator 1003 compares the phase of the clock signal 14 output from the crystal oscillator 1007 and divided by M with the phase of the video clock 15 output from the voltage controlled oscillator 1005 and divided by N.
A low-pass filter 1004 receives a comparison result from the phase comparator 1003 and changes an input voltage of the voltage controlled oscillator 1005. For example, if the phase of the clock signal 14 output from the crystal oscillator 1007 and divided by M advances from the phase of the video clock 15 output from the voltage controlled oscillator 1005 and divided by N, the input voltage to the voltage controlled oscillator 1005 is raised to advance the phase of the video clock 15.
The PLL 1008 has the 1/M frequency divider 1002, phase comparator 1003, low-pass filter 1004, voltage controlled oscillator 1005 and 1/N frequency divider 1006.
The operation of PLL will be described more specifically.
The clock signal 14 output from the crystal oscillator 1007 and divided by M and the video clock signal 15 divided by N are input to the phase comparator 1003. An output of the phase comparator 1003 is passed through the low-pass filter 1004 which supplies a voltage to the voltage controlled oscillator 1005. For example, if the phase of the clock signal 14 output from the crystal oscillator 1007 and divided by M advances from the phase of he video clock 15 divided by N, the input voltage to the voltage controlled oscillator 1005 is raised to advance the phase of the video clock 15.
If the frequency of the clock signal 14 output from the crystal oscillator 1007 is represented by fin, and that of the video signal 15 is represented by fout, then the following equation is satisfied:
fout=finxc3x97N/M
The values of N and M are adjusted in accordance with a detected main scan width to thereby adjust the video clock frequency and correct the main scan width.
In order to speed up an image forming operation, a plurality of lines are scanned at the same time by using a plurality of beams.
A scanner optical system capable of scanning a photosensitive drum with a plurality of beams, particularly with two beams, will be described briefly.
FIG. 2 is a perspective view showing the outline structure of a scanner optical system capable of scanning a photosensitive drum with a plurality of beams, particularly with two beams.
In FIG. 2, laser sources (semiconductor laser) 81 (81a, 81b) emit a plurality of laser beams (hereinafter simply called beams) 87a and 87b. A collimator lens 82 collimates the plurality of beams 87a and 87b output from the laser source 81. A polygon mirror 83 scans a plurality of beams 87a and 87b collimated by the collimator lens 82. An fxcex8 lens 84 adjusts the scan speeds of the plurality of beams 87a and 87b scanned by the polygon mirror 83. The plurality of beams 87a and 87b scanned via the fxcex8 lens form latent images corresponding to the video signals on the surface of a photosensitive drum 1. A position detection sensor (hereinafter called a BD sensor) 86 detects the plurality of scanned beams 87a and 87b and outputs horizontal synchronization signals (BD signals (BD(A), BD(B)).
The operation of the scanner optical system will be described more specifically.
The plurality of beams 87a and 87b emitted from the laser source 81 (81a, 81b) are collimated by the collimator lens 82 and thereafter scanned by the polygon mirror 83. The scan speeds of the plurality of scanned beams 87a and 87b are adjusted by the fxcex8 lens 84. Latent images corresponding to video signals are eventually formed on the photosensitive drum 1.
Scanning the photosensitive drum 1 with the plurality of beams as shown in FIG. 2 is associated with some problem. Because of a difference of an emission light wavelength between semiconductor lasers (in FIG. 2, semiconductor lasers 81a and 81b) of the laser sources, the main scan magnification for each beam becomes different (JP-A-06-227037). To solve this problem, a method of adjusting the main scan magnification for each beam by changing the video clock frequency for each beam has been proposed (JP-A-06-227037).
FIG. 3 is a block diagram showing the structure of video clock frequency variable unit of an image forming apparatus described in JP-A-06-227037. In FIG. 3, like elements to those shown in FIG. 2 are represented by using identical reference symbols.
In FIG. 3, crystal oscillators 91a and 91b output clock signals whose frequencies were adjusted so that each beam 87a, 87b has the same main scan magnification. Horizontal synchronization units (BD synchronization units) 92a and 92b synchronize the clock signals output from the first and second crystal oscillators 91a and 91b with BD signals 95 output from the BD sensors 86.
Laser drive units (LDA driver unit, LDB driver unit) 93a and 93b drive the semiconductor lasers (LDA 81a, LDB 81b) in accordance with video signals. As shown in FIG. 2, a single laser scanner optical system has two semiconductor lasers (LDA 81a, LDB 81b), and the main scan magnification for each laser beam can be adjusted independently.
The operation of the video clock frequency variable unit will be described specifically.
The first and second crystal oscillators 91a and 91b output clock signals whose frequencies were adjusted so that each beam 87a, 87b has the same main scan magnification. The clock signals output from the first and second crystal oscillators 91a and 91b are input to the horizontal synchronization units (BD synchronization units) 92a and 92b which supply the video clock signals synchronized with the BD signals 95 output from the BD sensors 86 to the laser drive units (LDA drive unit 93a, LDB drive unit 93b). The laser drive units (LDA drive unit 93a, LDB drive unit 93b) drive the semiconductor lasers (LDA 81a, LDB 81b) in accordance with video signals.
The main scan magnification correction of one color, e.g., black (k). has been described above. As shown in FIG. 3, similar units for other three colors (cyan (c), magenta (m) and yellow (y)) are also provided, totalling in four color units.
This configuration is, however, associated with the following problems. The image forming unit of each color including black (b), cyan (c), magenta (m) and yellow (y) has a plurality of laser sources (semiconductor lasers (LDA 81a, LDB 81b). It is therefore necessary to provide a number of crystal oscillators. In the example shown in FIG. 3, two crystal oscillators 91a and 91b are provided for each color, totalling in eight crystal oscillators for four colors. For example, if each of four image forming units scans four beams, sixteen crystal oscillators are necessary.
As described in JP-A-06-227037, if the common video clock signal is used for a plurality of semiconductor lasers, a refraction angle of the lens may become different for each laser beam because of a different wavelength of each laser beam generated by the semiconductor laser. Therefore, the main scan magnification for each laser beam becomes different and the formed image has a low precision.
For example, JP-B-06-57040 describes that if the video clock is to be changed by 0.1%, the counter for a frequency divider requires 10 bits, and that the minimum variable frequency of a video clock signal is determined by a frequency division ratio. Namely, N/M=1000/1000 is changed to N/M=999/1000 to obtain a variable step of xe2x88x920.1%.
According this teaching, if the video clock is to be changed by 0.001%, the frequency division ratio is set to about 100000. For example, in the case of xe2x80x9cmain scan width of reference color=300000 xcexcm (300 nm)xe2x80x9d, xe2x80x9cmain scan width of adjustment color=300003 xcexcmxe2x80x9d and xe2x80x9cN/M of adjustment color=1xe2x80x9d, then the N/M of adjustment color is set to xe2x80x9cN/M=0.99999 (corresponding to xe2x88x920.001%), e.g., N=99999 and M=100000xe2x80x9d.
In the case of xe2x80x9cmain scan width of reference color=300000 xcexcm (300 nm)xe2x80x9d, xe2x80x9cmain scan width of adjustment color=299997 xcexcmxe2x80x9d and xe2x80x9cN/M of adjustment color=1xe2x80x9d, then the N/M of adjustment color is set to xe2x80x9cN/M=1.00001 (corresponding to +0.001%), e.g., N=100001 and M=100000xe2x80x9d.
However, as N is made lager, the loop feedback amount of PLL becomes smaller. As in the examples described above, if N is nearly equal to 100000 and fout is nearly equal to 20 MHz, the feedback to the phase compartor 1003 is executed once per about 5 msec.
If jitter of VCO 1005 is large, jitter becomes conspicuous during the period while the feedback is not executed.
Since the feedback to the phase compartor 1003 is executed once per 5 msec, the jitter of the video clock signal 15 is improved only every 5 msec. While the jitter is not improved, the jitter of VCO 1005 itself appears. The jitter of a PLL circuit can be reduced to about the same degree of a reference clock. However, if N is set to about 100000, the feedback amount becomes small so that the jitter becomes large as compared to that of the reference clock.
As above, the conventional image forming apparatus has the following disadvantages.
If the main scan magnification variable step is made small in order to improve the correction precision of a main scan magnification, it is necessary to set larger the values of M and N of the frequency dividers 1002 and 1006 shown in FIG. 1. As the value of N is made large, the feedback amount of PLL becomes small so that the jitter of VCO 105 directly appears on the video click signal 15. The jitter of the video clock signal 15 results in the position misalignment or color aberration of the final image.
In order to reduce the jitter on the video click signal 15, a VCO having a small jitter may be used with a PLL circuit. However, in this case, the cost of the apparatus rises.
It is an object of the invention to solve the above-described problems.
It is another object of the present invention to form an image of high precision by correcting a difference of a scan magnification caused by a difference of a wavelength of each of a plurality of light beams with which the image is formed.
It is another object of the invention to form an image of high precision inexpensively and at high speed.
Under the above objects of the invention, according to one aspect of the present invention, there is provided an image forming apparatus comprising: an image forming unit including an image bearing member, beam generating means for generating a plurality of light beams and an optical scan system for scanning the image bearing member with the plurality of light beams generated by the beam generating means to write an image; beam detecting means for detecting the plurality of light beams scanning the image bearing member at predetermined positions and obtaining a plurality of beam detection signals; clock generating means for generating a single clock signal; control means for controlling the frequency of the single clock signal by controlling the clock generating means; and synchronization means for controlling the phase of the single clock signal generated by the clock generating means in accordance with the plurality of beam detection signals obtained by the beam detecting means, and generating a plurality of control clocks, wherein the generating means generates the plurality of beams in accordance with the plurality of control clocks.
The other objects and features of the invention will become apparent from the following detailed description of embodiments when read in conjunction with the accompanying drawings.